Control of agricultural pests has relied mainly on the use of chemical insecticides. The use of such man-made insecticide presents a number of technical and social issues that include a) development of insect resistance in the field, b) threat to beneficial microbial flora in the soil, c) human health hazards, and d) increase in the environmental burden.
Bacillus thuringiensis (Bt) is a gram positive ubiquitous soil bacterium characterized by its ability to produce crystalline inclusions during sporulation. The ingested Bt crystal protein is hydrolyzed to an active toxic molecule that binds covalently to the brush border membrane vesicles of the target larvae leading to creation of holes in the gut and/or osmotic imbalance with eventual death of the larvae. There is a high degree of host specificity for the various Bt pesticidal proteins, and to date no adverse effect on mammals including human beings/beneficial microbial flora has been reported. For these reasons, Bt has been considered a positive and effective alternative to chemical insecticides for many applications.
Problems can arise as new insect pests become endemic, however, or as existing populations develop resistance to a particular level or type of Bacillus thuringiensis toxin. Bacillus thuringiensis has been shown to be effective predominantly but not exclusively against Lepidopteran, Dipteran, and Coleopteran larvae. Cry proteins (d-endotoxins) from Bacillus thuringiensis have potent insecticidal activity against a number of these insects. These proteins are classified Cryl to CryV based on amino acid sequence homology and insecticidal activity.
Most Cryl proteins are synthesized as protoxins (ca. 130-140 kDa) then solubilized and proteolytically processed into active toxin fragments (ca. 60-70 kDa).
A large number of agronomically important pests are from the Lepidopteran, Dipteran, and Coleopteran insects. Sucking insects, however, present an important alternate class of pests that destroy host plants, not only through direct invasion but also as carriers of devastating viruses which are transferred into the host plant when the insect sucks fluid from the plant phloem. For example, whitefly (Bemisia tabaci) damages cotton plants by direct invasion and is also the carrier of leaf-curl virus, which attacked cotton in Sudan in the 1980's and in Pakistan in the early 1990's.
To date there are no known chemicals/biological agents that can effectively control the spread and infection of plant viruses, and for this reason there is a continuing need for new methods to control the virus vectors and virus carriers.
Recently, some reports have extended the host range of Bt to nematodes, fleas, cockroach, and aphids (1994 U.S. Pat. Nos. 05,350,577; 05,322,932; 05,281,530; 05,378,460; 05,350,5576; 05,302,387; 05,350,5576; and PCT/U.S. 93/07409). Despite the existence of these reports, few applications for using Bt have been developed other than for use against Lepidopteran, Dipteran, and Coleopteran larvae.
For all of these reasons, there is a particular need for new forms of the Bacillus thuringiensis toxin for use in protecting plants, a need which will only increase with time. More particularly, there is a continuing need to introduce newly discovered or alternative Bacillus thuringiensis genes into crop plants.